Elm guide random, now with more dices. https://guide.elm-lang.org/architecture/effects/random.html

random.elm

import Html exposing (..)
import Html.App as App
import Html.Attributes exposing (..)
import Html.Events exposing (onClick)
import Random

main =
  App.program
    { init = init
    , update = update
    , subscriptions = subscriptions
    , view = view
    }

-- MODEL

type alias Model =
  { dieFaces : List Int
  }

-- INIT

init : (Model, Cmd Msg)
init =
  (Model [1], Cmd.none)

-- UPDATE

type Msg
  = Roll
  | NewFaces (List Int)
  | NewDice

update : Msg -> Model -> (Model, Cmd Msg)
update msg {dieFaces} =
  case msg of
    Roll ->
      ( Model dieFaces
      , Random.generate NewFaces (Random.list (List.length dieFaces) (Random.int 1 6))
      )

    NewFaces newFaces ->
      (Model newFaces, Cmd.none)

    NewDice ->
      (Model ( 1 :: dieFaces ), Cmd.none)


-- VIEW

view : Model -> Html Msg
view model =
  div []
    [ ul []
      (List.map showDice model.dieFaces)
    , button [ onClick Roll ] [ text "Roll" ]
    , button [ onClick NewDice ] [ text "Add" ]
    ]

showDice dice =
  li [] [ text (toString dice) ]

-- SUBSCRIPTIONS

subscriptions : Model -> Sub Msg
subscriptions model =
  Sub.none